What do most complex organisms lack in terms of tissue structure?

Most complex organisms, particularly animals, lack cell walls, which are rigid structures found in plants, fungi, and some bacteria. Cell walls provide support and protection to cells, but in complex organisms such as animals, cells are instead surrounded by flexible cell membranes. This flexibility allows for a greater range of movement and interaction between cells, which is crucial for the complex functions and dynamics within multicellular organisms.

Additionally, the absence of cell walls contributes to the diversity of cell types and functions found in complex organisms, enabling tissue specialization and sophisticated organ systems. For instance, muscle cells need to contract, and nerve cells need to transmit signals, both of which are more easily accomplished with membrane-bound cells that can change shape and communicate freely with one another.

In species with cell walls, like plants, the necessity for rigidity is met through that structure, which enables the plant to maintain its form and withstand environmental pressures. However, in complex organisms, the lack of such a structure facilitates more dynamic interactions among cells and tissues, important for functions like growth, adaptation, and mobility.